(1) Field of the Invention
The present invention relates in general to the field of intrusion detection, and in particular to detection by interruption of wave beams.
(2) Description of Related Art
A significant need exists to detect individuals who cross a boundary into an area that they are not allowed to enter. Nations have a great need to prevent the infiltration across their borders by agents of terrorist organizations or of other hostile nations. The need also exists to apprehend aliens who attempt to enter a nation to gain economic benefit without going through the lawful immigration process. The perimeters of many facilities, with examples being; military bases, military test ranges, penal institutions, and nuclear power plants, all must be monitored to assure that no individual enters or leaves the facility in other than an authorized manner. Both national borders and facility perimeters are often protected by such installations as woven wire fences or solid walls. However, a determined intruder can surmount or penetrate these structures, and thus additional detection measures are needed.
A desirable method of detecting the presence of an intruder is the establishment of an “electronic wall” that is formed by a multiplicity of volumes wherein any intruder entering one or more of these volumes will be detected. These volumes must have clearly defined limits to minimize false alarms from targets such as individuals or vehicles transiting near but not passing through the electronic wall.
An electronic wall can be used in conjunction with other barriers. For example; woven wire, or similar construction, fences can be positioned to one or both sides of and parallel to the electronic wall, to form a highly effective perimeter protection system for a facility or national border. The physical fence is typically fabricated of woven wire or similar material, and has sufficient height and wire gage that climbing over or cutting through the fence will require the expenditure of considerable time and effort. A preferred arrangement is the positioning of the electronic wall so that it must be penetrated before the intruder arrives at the physical fence. When the intruder passes through the electronic wall a detection message is immediately sent to the controlling authority, informing them that at a defined location a passage has occurred through the electronic wall. Interdiction personnel can be dispatched to the defined location to apprehend the intruder, often before the intruder can achieve penetration through the physical fence.
Prior art examples of electronic walls or fences for intruder detection have used micrometer, millimeter or infrared wavelengths in the electromagnetic spectrum. Various methods of generating single or multiple beams as well as methods of detection have been used.
Prior art infrared intrusion detectors use such methods as the formation of a beam or beams traversing the detection area with the breaking of a beam by the passage of an intruder initiating a detection, or collecting the reflection of the beam off the intruder by a receiver as a means of detection. Still another method comprises a passive system that detects the difference in temperature between the ambient environment and the intruder. The usefulness of the infrared class of prior art is limited by atmospheric conditions including heavy rain and fog that may interrupt the beams, and by the growing availability of infrared viewing equipment that may make infrared beams visible to a well equipped intruder.
Some examples of prior art that operate in the microwave and millimeter-wave spectrum are capable of generating confined beams to produce an electronic detection zone. Some place a transmitter at one end of the protected volume and a receiver at the other to form a beam or detection volume between the two. The patent, Blacksmith, et. al., U.S. Pat. No. 4,132,988, issued on Jan. 2, 1979, uses this configuration with a plurality of passive reflectors to surround a rectangular area and to place the receiver near the transmitter. The passive reflectors are used to change the direction of propagation of the beam, typically by 90 degrees, several times to surround the area to be protected and return the beam to the transmitter/receiver location. Interruption, or breaking, of the beam initiates an alarm. In this and similar systems a single beam is used with a cross section defined by the directivity of the antennas and the dimensions of the passive reflectors. An astute intruder could devise means to go under or over the beam and thus avoid detection.
Other prior art sensors based on radar concepts generate a single or multiple beams that are confined in azimuth and elevation, and seek to detect an intruder within one or more of the beams by ranging on the radar return from the intruder. These sensors require significantly greater emitted energy than beam breaker systems.
The patent, Bjornholt et. al., U.S. Pat. No. 6,466,157, issued on Oct. 15, 2002, generates an electronic fence using a high-resolution millimeter-wave radar in conjunction with multiple passive reflectors that reflect the millimeter-wave energy back toward the radar to produce a multiplicity of electromagnetic beams. A sensor structure is located at one end of the electronic fence volume while a reflector structure is positioned at the opposite end at a distance of some two or three hundred meters. The sensor structure includes a radar that is sequentially coupled to three radar antennas positioned various heights. One antenna produces a beam broad in elevation to detect the radar return from the person of any intruder infiltrating the volume of the fence in near proximity to the sensor structure. Multiple beams are formed between a second radar antenna and passive reflectors placed along the floor of the electronic fence in the mid-region. Additional multiple beams are formed between the third radar antenna and passive reflectors of two types located in the more distant portions of the electronic fence. The first type of passive reflector is identical to that used in the mid-region, while the second type comprises an antenna coupled to a length of waveguide that is shorted at its distal end. Millimeter-wave energy enters the antenna, travels down the waveguide, is reflected, and after returning through the waveguide is reradiated by the antenna. Each passive reflector of this second type has a unique length of waveguide to produce an apparent different radar range from the radar in the sensor structure. Each radar beam included in the electronic fence has a different apparent radar range; thus the radar signal processing circuitry is capable of determining which beam has been interrupted by an intruder and can estimate the location of the intrusion.
The prior art design of Bjornholt, et. al., exhibits several disadvantages. These include the use of three different radar antennas and the resulting necessity to couple the radar to these antennas sequentially one at a time. The radar signal processing circuitry must process the data from each antenna independently and then integrate the result into a single output. Another disadvantage is the amount of signal loss that occurs in the waveguides of the passive reflectors included in the reflector structure.
Most prior art electronic walls comprise an insufficient number of electromagnetic beams that cover only parts of the length and height of the wall, thereby allowing “holes” for intruders to pass through. In addition, the prior art intrusion detectors are complex in design, difficult to transport, consume a great amount of prime power, emit high levels of electromagnetic energy, have a low probability of detection of any intruder traversing the electronic wall, and have a high false alarm rate for any entity even near (but not passing through) the wall.